1. Field of the Invention
The present invention is in the field of gamma ray testing of geological formations. In particular, the invention delineates boundaries of a formation in a highly deviated borehole from recorded spectra.
2. Description of the Related Art
Well logging systems have been utilized in hydrocarbon exploration for many years. Such systems provide data for use by geologists and petroleum engineers in making many determinations pertinent to hydrocarbon exploration. In particular, these systems provide data for subsurface structural mapping, defining the lithology of subsurface formations, identifying hydrocarbon-productive zones, and interpreting reservoir characteristics and contents. Many types of well logging systems exist which measure different formation parameters such as conductivity, travel time of acoustic waves within the formation and the like.
One class of systems seeks to measure incidence of nuclear particles on the well logging tool from the formation for purposes well known in the art. These systems take various forms, including those measuring natural gamma rays from the formation. Still other systems measure gamma rays in the formation caused by bursts of neutrons into the formation by a neutron source carried by the tool and pulsed at a preselected interval. The natural gamma ray log is particularly useful since radioactivity in earth formations is primarily due to potassium in shales and clays, and thus serves as a lithology indicator.
In general, one such method and apparatus, when applied to the field of natural gamma ray well logging, has utilized channels of spectra including those of potassium-40, uranium and thorium. One such technique utilizes these natural gamma ray logs to determine characteristics of shale formations as described in U.S. Pat. No. 4,071,755, to Supernaw et al. Unfortunately, it can be difficult to differentiate such shale formations from other formations having relatively high concentrations of uranium, such as uranium-rich sands or formations exhibiting uranium oxide plating as a result of fluid movement.
U.S. Pat. No. 4,612,439 to Chace provides methods and apparatus for evaluating subsurface formations utilizing natural gamma ray spectral logging. A high-resolution, gamma ray spectrometer incorporated in a well logging instrument traverses a borehole, whereby natural gamma radiation strikes a scintillation crystal contained therein. The detected gamma rays striking the crystal cause the crystal to emit photons in the visible energy region, the intensity of which is proportional to the energy lost in the crystal by the incident gamma ray. Light energy from the crystal is optically coupled to a photomultiplier tube where the energy is converted to a proportional electrical pulse signal which is amplified and transmitted to processing circuitry.
There is extensive prior art in the use of resistivity measurements for the detection of bed boundaries and distances to bed boundaries in highly deviated boreholes. See, for example, U.S. RE35386 to Wu et al. U.S. Pat. No. 6,308,136 to Tabarovsky et al. teaches a method for interpretation of induction logs in near horizontal boreholes. One of the problems in interpretation of resistivity logs is the shoulder bed effect wherein “horns” develop in the tool response at bed boundaries. Complicated processing has to be done to correct for the shoulder bed effects. U.S. Pat. No. 5,184,079 to Barber discloses dip-dependent filtering that may be applied to induction logs to produce corrected resistivity measurements. All of the methods discussed above have problems with identification of thin beds.
Resistivity measuring devices of the type discussed above are active devices in that an electrical signal is produced by a transmitter on a downhole logging tool and a receiver detects signals that are responsive to the formation properties. It would be desirable to have a passive method and apparatus that may be used for detection of bed boundaries, particularly of thin beds. The present invention satisfies this need.